Semiconductor devices are most often made by chemically and physically processing silicon wafers, and pick-up tools using vacuum suction are often used for handling the silicon wafers, instead of more conventional pincers. Such pick-up tools are required to be resistant to various chemicals and high temperatures which are used to etch or otherwise process the silicon wafers, and to be free from electrostatic charges because electrostatic charges can damage the silicon wafers. Examples of such pick-up tools are disclosed in Japanese patent publication (kokoku) No. 3-50672, and Japanese patent publication (kokoku) No. 5-75554.
A pick-up tool is equipped with a valve unit for controlling the vacuum suction, and the valve unit also must meet the above-mentioned requirements which are associated with the handling of silicon wafers. The valve unit typically includes a valve main body having a valve seat defined therein, and a valve member which is slidably received in the valve seat so that a passage defined inside the valve main body may be selectively opened and closed as required by turning or otherwise moving the valve member in relation with the valve seat. The valve member must be closely fitted in the valve seat so that a required fluid-tightness may be obtained, and it is essential that the valve main body has a sufficient rigidity not to deform and to maintain the fluid-tightness under substantial external forces. The material for the valve unit must be also capable of withstanding various chemical substances, moisture and temperatures of up to 250.degree. C.
The requirements for the material of the valve unit can thus be summarized as given in the following.
(1) The material should be electroconductive so as to avoid electrostatic charging of the valve unit components which could cause damages to the silicon wafers and lead to deposition of foreign matters. PA0 (2) The material should be resistant to chemical substances which are used for chemically processing and washing the silicon wafers, and should not release any metallic ions when subjected to such chemical substances. PA0 (3) The material should be self-lubricating so that smooth operation may be ensured and emission of particles may be avoided even after long use.
One of the most preferred materials for such valve units as well as such article pick-up tools is fluoride resins such as tetrafluoroethylene. Fluoride resins can meet most of the required properties, but are relatively readily deformable. Therefore, when subjected to significant external forces, a valve unit made of a fluoride resin may not be able to operate smoothly or may lose fluid-tightness. In particular, the screws used for securing a fluoride resin member may become loose in time due to the combined effect of repeated application of fluid pressure and the lack of the mechanical strength of the fluoride resin material. Loosening of the screws, which are typically made of metal or alloy, is also a cause of the rusting of the screws because it increases the chance of exposing the screws to various chemical substances. Additionally, fluoride resins are less immune to electrostatic charging than other resin materials, and the article pick-up tool could become undesirably electrically charged as it is handled by hand.
Fluoride resins such as tetrafluoroethylene are also known to be unsuitable for injection molding and extruding, and are therefore not suited to be molded into complicated shapes.